The metals which are used on the skin of hypersonic flight vehicles are susceptible to oxidative and thermal degradation during flight. The skin is exposed high temperatures and low pressures which cause unprotected metal surfaces to oxidize and volatilize during flight, leading to rapid failure. It is desirable to construct a skin which possesses high emittance for high reradiation and low volatility to prevent loss of the skin during flight. Refractory metals, such as columbium and molybdenum, are typically used; however, these metals are still susceptible to oxidative and thermal degradation.
In addition to being exposed to high temperatures and low pressures during flight, the leading regions of hypersonic flight vehicles are exposed to an ionized gas stream with dissociated species of atomic oxygen and atomic nitrogen. Uncoated refractory metals catalyze the recombination of the dissociated gases on the metal surfaces during hypersonic ascent and reentry of the flight vehicle. The heat of reaction produced by the recombination of the dissociated species further increases the temperature of the skin, and therefore, increases the probability of failure.
In the prior art, others have developed coatings which are applied to the metal skin to protect the metal from oxidation. Such coatings include intermetallic and silicide-based coatings, such as R512E, or Hf--Cr--Si. These prior art coatings have proven only partially successful in protecting the skin from surface oxidation, and there has been no showing that the coatings protect the metal surfaces from catalytic recombination. Furthermore, prior art coatings are also known to volatilize in the low pressure and high temperature environments during flight. If the coating volatilizes, the metal is left unprotected, and major repair must be undertaken, which is costly and time-consuming. It is, therefore, desirable to develop a coating having high emittance, and which protects the metal skin, particularly the leading edges, from degradation, and exhibits low catalytic recombination performance. It is also desirable to develop a facile, inexpensive method for maintaining the silica protective coating.